Recent technological advances have led to complementary metal-oxide-semiconductor (CMOS) sensor imagers being leveraged by cameras, video systems, and the like. CMOS sensor imagers can include an integrated circuit with an array of pixel sensors, each of which can comprise a photodetector. Moreover, a CMOS sensor imager can be incorporated into a System-on-Chip (SoC). As such, the SoC can integrate various components (e.g., analog, digital, . . . ) associated with imaging into a common integrated circuit. For instance, the SoC can include a microprocessor, microcontroller, or digital signal processor (DSP) core, memory, analog interfaces (e.g., analog to digital converters, digital to analog converters), and so forth.
Visible imaging systems implemented using CMOS imaging sensors can reduce costs, power consumption, and noise while improving resolution. For instance, cameras can use CMOS imaging System-on-Chip (iSoC) sensors that efficiently marry low-noise image detection and signal processing with multiple supporting blocks that can provide timing control, clock drivers, reference voltages, analog to digital conversion, digital to analog conversion and key signal processing elements. High-performance video cameras can thereby be assembled using a single CMOS integrated circuit supported by few components including a lens and a battery, for instance. Accordingly, by leveraging iSoC sensors, camera size can be decreased and battery life can be increased. Also, dual-use cameras have emerged that can employ iSoC sensors to alternately produce high-resolution still images or high definition (HD) video.
A CMOS imaging sensor can include an array of pixel cells, where each pixel cell in the array can include a photodetector (e.g., photogate, photoconductor, photodiode, . . . ) that overlays a substrate for yielding a photo-generated charge. A readout circuit can be provided for each pixel cell and can include at least a source follower transistor. The pixel cell can also include a floating diffusion region connected to a gate of the source follower transistor. Accordingly, charge generated by the photodetector can be sent to the floating diffusion region. Further, the imaging sensor can include a transistor for transferring charge from the photodetector to the floating diffusion region and another transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference. Moreover, three signals can be provided to each pixel cell in the pixel array: a transfer (TX) signal, a reset signal, and a select signal.
The array of pixels cell can operate in a variety of modes. For instance, rolling shutter operation can be utilized to readout a single row of pixels from the pixel array at a particular time. According to another example, a global shutter can be employed to readout all rows (or substantially all rows) of pixels from the array at a given time. To enable readout from the array, conventional techniques oftentimes use a booster to boost signals provided to each pixel in the array; more particularly, the booster can be utilized to increase voltages above Vdd, which is a positive supply voltage, and/or decrease voltages below Ground for transfer signals and/or reset signals provided to pixels in the array. However, commonly employed boosters can lack sufficient speed for reading out pixels from the array when operating in rolling shutter mode or global shutter mode. Conventional boosters can also introduce row-to-row variation of boosted voltage levels. Moreover, typical boosters oftentimes are unable to drive large loads that are commonly encountered when operating in global shutter mode.